Add baseline unit tests and PR CI for bot utilities + manager auth

[Copilot]

The repo had no test suite and PRs to main did not run tests. This introduces initial coverage and a PR workflow that executes the test suites.

• Python utilities: added unittest coverage for LLM helpers, chat message shaping, emoji/sticker replacement, cache behavior, and singleton behavior.
• Go manager: added auth middleware test coverage for missing/invalid/valid bearer tokens.
• CI: new PR workflow runs Python and Go tests with read-only permissions.

Example test coverage:

result = prepare_chat_messages(prompt="describe", image=b"image")
assert result[0]["content"][1]["image_url"]["url"].startswith("data:image/jpeg;base64,")

Original prompt

Go through the entire codebase and write tests. When a PR is created (to merge to main, as pushing to main is blocked), it should run that test suite in GitHub workflow.

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[kashifulhaque]

LGTM